The stamp issued in 1981 was issued to commemorate the 15th century of the Hegira, Mohammed’s the departure from Mecca in September 622. If this were the 15th normal century we would be looking at 2122. However, Islam uses a lunar calendar and dates from the year of the Hegira. So, 1981 in the ordinary calendar is 1401 in the Islamic lunar calendar, and the stamp commemorates the 15th lunar century of the Hegira.